There are ways on how we can link multiple accounts in QuickBooks, uber-lyfttax2018.
If you're referring to your QuickBooks Self-employed account, we'll have to make sure that your User ID and Password are the same as the other one.
If you're trying to link multiple bank accounts, we'll have to connect the two-accounts into QuickBooks. Then, make sure that they have the same credentials. This way, the system will automatically sync the transactions of the two accounts.
You can check this article for more information when connecting a bank account: Connect bank and credit card accounts.
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